1. Field of the Invention
This invention relates generally to a junction device for a plurality of flows, and more specifically to an anatomical or surgically created connection.
2. Description of Related Art
The incidence among other congenital heart defects of children born in the USA with complex congenital heart defects in which there is only one effective pumping chamber is about 20%. Some studies indicate that the overall incidence of congenital heart defects is 1%—and therefore, generally two babies per 1000 births will be born with a single ventricle. If untreated, life expectancy can be short.
Surgical repairs that separate the pulmonary and systemic circuits, placing them in series with the univentricular pump, termed “Fontan Repairs,” are palliative, and unfortunately not curative. Operations are routinely staged over many years and survivors often require a lifetime (rather limited) of intensive medical attention. Cardiologists report that their patient populations with this complex cardiovascular physiology (approximately 20%) require a disproportionate share of their time (at least 50%).
The concept of a total right ventricular bypass, first introduced by Fontan and Baudet in 1971, is a palliative procedure aimed at separating the systemic and pulmonary circulations, thus eliminating mixing of oxygenated and deoxygenated blood. The remaining left ventricle drives the blood flow throughout the entire body.
Since its inception, modifications of the Fontan procedure have steadily improved surgical outcomes, reducing the post-operative mortality to the level of more simple types of congenital heart disease repairs. However, the marked improvement in surgical outcome is balanced by the numerous and serious long-term complications encountered by the Fontan patients, such as ventricular dysfunction, thromboembolism, arrhytmias and protein loss enteropathy.
As a general background, in the cardiovascular system, blood is a major means of transportation for the nutrients and wastes that travel to and from the body's tissues. It is pumped through the entire body by the heart, and then perfuses each single tissue through a complex network of arteries, capillaries and veins. The cardiovascular circulation can be subdivided into two primary circuits: the pulmonary and systemic circulations. The pulmonary circuit describes the path going from the heart to the lungs and back, and the systemic circulation transports the blood between the heart and the remainder of the body.
The normal heart has four chambers: the left and right atria and the left and right ventricles. In a normal physiology the septum separates the left and right sides of the heart creating two distinct pumps that function in parallel. The left side of the heart drives the blood through the systemic circuit, while the right side drives the blood through the pulmonary circuit.
This four-chambered structure of the heart is essential to its function. The ventricles provide the pumping force, while the atria provide the buffer volume needed to receive the continuous blood flow returning from the body or the lungs. In addition to these four chambers, four valves control the inlet and outlet of both ventricles to prevent blood-flow reversal and ensure the efficiency of the ventricular contraction. When the left ventricle contracts during systole, the increase in ventricular pressure closes the bileaflet mitral valve and forces the trileaflet aortic valve open. Consequently, most of the blood that was present in the left ventricle before systolic contraction must flow from the left ventricle through the open aortic valve into the aorta then to the rest of the body.
Meanwhile, the blood returning from the lungs through the pulmonary veins is stored in the left atrium. As pressure builds up in the left atrium and in the aorta and decreases in the left ventricle during diastole, the mitral valve reopens and the aortic valve closes. Blood then flows from the left atrium through the mitral valve into the left ventricle.
Similarly, the systemic blood coming back from the body flows through the inferior vena cava (IVC) and superior vena cava (SVC) into the right atrium. It then passes through the tricuspid valve into the right ventricle from where it is discharged through the trileaflet pulmonary valve into the pulmonary circulation for gas exchange.
Congenital heart defects (CHDs) describe all abnormalities of the heart or of the great arteries (pulmonary arteries and aorta) that are present at birth. They are the leading cause of infant mortality in the western world accounting for about 20% of all infant death. CHDs alone account for one-third of all birth defects affecting one in every 100 infants in the United States. They are the number one cause of birth defect related deaths during the first year of life, and the mortality of these children may be as high as 50% depending on the condition.
CHDs arise from faulty embryogenesis between the third and eighth week of gestation, when major cardiovascular structures develop—going from a simple straight tube to a complex four-chambered heart with separate pulmonary and systemic circuits. Over 35 different forms of CHDs have been reported, and although the exact cause of CHDs is still unknown in most cases, multifactorial genetic and environmental parameters, including chromosomal defects, viruses, chemicals and radiation are suspected.
Among all CHDs, particularly challenging are the defects (or combination of defects) observed in about 20% of the CHD cases that effectively lead to a single ventricle (SV) anatomy. This physiology results in communication between the systemic and pulmonary circulation, thereby eliminating the two-pump system and allowing for the mixing of oxygenated and deoxygenated blood. It is well understood that venous blood mixing should be avoided, and corrected if possible.
FIG. 1 is a schematic showing differences between normal, single ventricle, and Fontan physiology. In a normal physiology, the vena cavae contains deoxygenated blood coming from the systemic circulation and the pulmonary arteries carries the deoxygenated blood from the right ventricle to the lungs for the blood to become oxygenated.
The single ventricle physiology pumps high pressure blood to both the systemic and the pulmonary circulation. The high pressure in the pulmonary circulation and the mixing of oxygenated and deoxygenated blood causes many of problems, which is why the Fontan procedure is performed. The current Fontan surgical procedure of choice for patients with single ventricle physiology is the total cavopulmonary connection (TCPC).
The most prevalent CHDs leading to a SV anatomy include multiple ventricular and/or atrial septal defects, tricuspid atresia, hypoplastic left or right heart syndrome, transposition of the great arteries, and a double inlet ventricle.
Without surgical intervention, survival of patients with blocked right or left heart pathways as a result of a transposition of the great arteries, a tricuspid atresia or an acute hypoplastic heart syndrome once depended on the presence of coexisting defects such as a septal defect or a patent ductus arteriosus. In the middle of the 20th century, surgical shunt procedures were developed as a palliative procedure for cyanotic CHD. The purpose was to connect the pulmonary arteries (PAs) with the systemic arteries or with the SVC so as to try and augment the pulmonary blood flow.
These shunts enabled short-term survival. However, ventricular dysfunction, pulmonary vascular disease, and chronic cyanosis prohibited a normal existence and drastically shortened patient life expectancies with only few patients surviving beyond adolescence.
The advent of the Fontan operation in 1971 brought about a revolution in the management of single ventricle heart defects. In a Fontan type circulation, the left ventricle pumps blood into the aorta and arteries. This blood flows at first rapidly into the different organs. The very same force pushes the blood across capillaries, and through the veins. But by its very nature, this flow depends on many factors. For instance, if the blood vessels in the lung are thick walled and narrow before surgery, they will offer very high resistance to passive blood flow. In such a state, the Fontan operation cannot be performed, or will have a high risk of failure, since the extra energy needed to maintain lung blood flow is not available.
Even normally some amount of resistance will exist across the lung blood vessels. After a Fontan operation, the pressure in the veins will therefore be higher than normal, in order to overcome this resistance and maintain lung blood flow. The elevated pressure in the veins has a few ill effects, including that there may be swelling of the entire body due to fluid from the blood leaking out of the vein walls, there may be facial puffiness, fluid accumulation in the abdomen (ascites) or chest (pleural effusion), and sometimes even absorption of nutrients from the intestines is affected.
Indeed, the heart may eventually fail. The age at which the heart fails and the patient requires a heart transplant depend on many different factors, but a main reason appears to be the excessive workload placed on the single ventricle. The major causes of death are: heart failure, arrhythmia, protein losing enteropathy, and embolisms.
The principle of a complete right heart bypass, where the systemic veins were directly connected to the pulmonary arteries without going through the single ventricle, achieved a number of salutary transformations to the SV anatomy. It re-separated the systemic and pulmonary circuits and eliminated venous blood mixing, which in turn ostensibly improved arterial oxygen saturation and patient color.
The original Fontan procedure included the construction of two independent VC-to-PA tracks, the IVC-to-LPA and SVC-to-RPA, with the anastomosis of the right atrium directly onto the PAs and a valve placed in the IVC.
However, it soon became clear that placing a valve in the caval conduits, rather than being advantageous, resulted in obstruction of the low-pressure VC-to-PA circulation. Furthermore, the separation of the IVC-to-LPA and SVC-to-RPA tracks, that had been designed to ensure an even perfusion of the right and left lungs, did not allow for any adaptation of the LPA/RPA blood flow ratio, leading to serious complications when one of the pulmonary tracks became obstructed. Additionally, such a cardiovascular configuration excluded all hepatic blood flow from the RPA, which was demonstrated to be strongly correlated with pulmonary venous malformations.
Shortly after the first successful right ventricular heart bypass operation for tricuspid atresia, a modified Fontan procedure was demonstrated wherein the entire venous return could be diverted to the pulmonary circulation through a single valveless atrio-pulmonary (AP) connection. This procedure had the combined advantages of providing the pulsatile action of the atrium, redistributing the hepatic fluid to both lungs and splitting the pulmonary blood flow depending upon the needs and resistance of either lung.
The valveless AP-connection was the first in a series of modifications of the original Fontan procedure. Although this procedure was quickly endorsed and has had widespread use in many centers, the long-term follow-up of patients with an AP-connection indicated that they were prone to late complications. Patients developed supraventricular arrhytmias, right atrial thrombus, exercise intolerance and other symptoms of low cardiac output. These complications were usually related to a markedly dilated right atrium appendage, which was suspected to be due to the increased pressure load imposed on the atrium. This atrial dilatation was in turn associated with stagnant flows along the dilated right side of the atrium and turbulent flows elsewhere in the connection, resulting in significant fluid energy dissipation.
The high incidence of right-atrium related complications led many to question the role of the pulsating right atrium and its actual contribution to the Fontan circulation. There was in vitro and in vivo evidence that the interposition of a passive chamber with impaired systolic function between the VCs and PAs was a major cause of flow inefficiency, and the total cavopulmonary connection was proposed as a logical alternative to the Fontan procedure.
The TCPC has been described as the anastomosis of the SVC directly onto the RPA followed by the creation of a tunnel through the right atrium connecting the IVC to the inferior aspect of the RPA. This geometry has been demonstrated to lead to more streamlined flow patterns with less turbulence and fluid energy loss when compared to the AP-connection. These findings were confirmed both by in vitro and computational fluid dynamic studies. Retrospective clinical studies also investigated early and late mortality rates. Findings show that the TCPC is accompanied by a lower mortality rate, improved outcomes and a more favorable course during the postoperative period.
Staging the operations has markedly improved surgical outcomes, and allowed the Fontan surgery to be applied to a larger range of SV-patients. It is now an integral part of the methodology for SV heart repairs. Usually, there are three different stages involved in the completed “Norwood procedure” leading to the resultant Fontan—each stage with inherent morbidity and associated mortality.
The first stage is commonly performed immediately after birth or within the first two weeks of life and is referred to as the first stage Norwood or Norwood I procedure. It involves creation of a systemic to pulmonary arterial connection, arch reconstruction and coarctation repair with anastamosis of the native aorta to the pulmonary arterial trunk (also called the Damus-Stancil-Kaye procedure) and an atrial septectomy.
A more recent modification, the “Sano” procedure, involves placement of a right ventricle to pulmonary arterial connection and elimination of the systemic to pulmonary arterial shunt.
The second stage bi-directional “Glenn” procedure (Norwood II), performed at approximately three to four months of age, involves anastamosis of the superior vena cavae to the pulmonary arteries and removal of the systemic to pulmonary shunt or the RV to PA shunt and has the lowest reported risk.
The last stage (Norwood III), or Fontan procedure is creation of a connection of the inferior vena cavae to the pulmonary arteries via an intra-atrial or extra-cardiac connection. This results in the total cavopulmonary connection.
Although most surgeons agree on the staged TCPC as being the current procedure of choice for Fontan repairs, controversies exist about the selection of the connection type, the type of material to use, the need for fenestration, and the timing of the operations.
The choice of connection type seems to be dictated by surgeon preference, each type with pros and cons. When compared to intra-atrial tunnels, extra-cardiac conduits provide numerous advantages including smoother geometries, fewer atrial suture lines thus minimizing sinus-node damage, and less or no time on the heart- lung machine. On the other hand they provide no growth potential and may lead to conduit stenosis and thromboembolism. Although long-term follow-ups are not yet available, early- and mid-term results for extra-cardiac conduits are favorable, especially combined with a fenestration in the inferior conduit.
Including a fenestration has been demonstrated to lower the systemic venous pressures as well as to improve ventricular filling, consequently leading to improved cardiac output and overall oxygen delivery. While some institutions advocate systematic fenestration, others argue that it should be used more selectively, balancing the potential benefits against the risks and costs of the additional intervention needed to close the fenestration.
Similarly, the material of choice varies from institution to institution and patient to patient. Intra-atrial tunnels have been built out of polytetetrafluoroethylene (PTFE) patches, pericardial patches and autologous pericardial patches. Extra-cardiac conduits have been constructed using PTFE, Dacron, and autologous pericardium flaps. Mid-term results have been favorable for all synthetic materials and only short-term follow-up data (30 months) is available for autologous conduits.
Finally, as to the timing of the operation, the mean age at TCPC completion and mean interval since previous palliation have significantly decreased over the past decade. While some see this as a beneficial trend that has reduced most of the major complications, others recommend caution pointing out that suture lines significantly limit vessel growth and that vessel size is a major factor for hemodynamic efficiency.
Thus, it can be seen that although various cardiac repair options are known, there is still much room for improvement. Indeed, this should not take away from the success of the Fontan procedure. It is simply that connecting the vena cavae directly to the lungs, bypassing the heart, is a difficult task for the single ventricle due to chronic work and volume overload.
In FIG. 2(a), the normal operating circulation with the two (left and right ventricles) pumping action of the heart is illustrated. The resistance and pressure values shown are representative average values for a healthy adult person measured clinically by catheterization. FIG. 2(a) is a schematic of normal circulation—lumped representation of pulmonary and systemic beds. FIG. 2(b) illustrates various congenital heart defects. The systemic vascular resistance (Rsys) is approximately 17.5 mmHg/L/min on average, and the pulmonary vascular resistance (Rpul) is approximately 1.8 mmHg/L/min on average.
As used in FIG. 2, pu is a pulmonary bed (lungs); sys is a systemic bed (upper and lower body combined); PDA is the patent ductus arteriosus; {circle around (x)} illustrates the flow restriction at aorta; arrows represent the blood flow directions and cardiac shunts; Rtri is the tricuspid valve resistance; Rpulv is the pulmonary valve resistance; Rmi is the mitral valve resistance; and, Rao is the aortic valve resistance.
Constant representative resistance values are used, although it will be understood by those of skill in the art that vascular resistances are regulated to some extent by the body in response to blood pressure, the changing oxygen saturation during exercise, and vary slightly with flow pulsatility. Further, newborn children have higher pulmonary resistance values.
An average blood flow rate (i.e. cardiac output) for normal adults at rest is 5 L/min. For children with the TCPC, the total cardiac output is closer to 3 L/min. Cardiac output can be as high as 20 L/min during exercise conditions for normal adults and in some athletes it goes even higher.
The resistances of the right and left hearts are mainly due to the resistances of the heart valves which are small (˜1.5 mmHg/L/min) compared to the vascular resistances unless there is valve stenosis or other problems.
As previously explained, one-fifth of congenital heart defects require a series of complex palliative surgeries which result in a single operating ventricle with the pulmonary and systemic beds arranged in series, as shown in FIG. 3. This configuration results in higher pressures in the systemic bed with severe clinical consequences. FIG. 3 illustrates single ventricle circulation after final stage Fontan surgery. RTCPC is the equivalent TCPC resistance calculated from in vitro experiments for an intra-atrial patient specific anatomy.
There are two versions of the TCPC namely, extra-cardiac and intra-atrial. Yet, current extra-cardiac and intra-atrial TCPC yields high energy losses in the blood flow because of the mixing of the blood. FIGS. 4 and 5 shows models of the TCPC with a transparent blood analog containing particles flowing through the connection. FIG. 4 illustrates an in vitro glass model of a “zero” offset TCPC. FIG. 5 illustrates an in vitro model of a one diameter offset and flared model of a Cavopulmonary Connection.
The two inlets (SVC and IVC) and the two outlets (RPA and LPA) are shown in different configurations. As can be seen from the two figures, there is a region of the connection where there is a high level of venous flow mixing and disturbance. Such a region exists as the inlets either present inlet flows at 180 degrees from one another, forcing the flows to collide head on, before separating toward the two outlets, or the inlets do not provide such a direct path of collision, but even with offset of some amount, the inlet flows nonetheless collide with each other or with a wall sufficiently to present outlet flows with marked energy loss.
The region of mixing in present connection designs negates the beneficial momentum present in the two inlet flows that exists before the blood in the IVC and SVC enter the connection site, wherein the momentum loses due to swirling, turbulence, and other collision characteristics, limits the outlet flows to the lungs.
In this mixing region of the connection, there is a lot of swirling of the flows, when the blood analogue from the inlets mix, which causes substantial energy loss, causing the outlet flow to lose momentum towards the lungs via the two outlets. This is known disadvantage with the current configurations used for Fontan procedures.
RPA/LPA flow splits of 30/70, 40/60, and 50/50 in the models of FIGS. 4 and 5 have shown that when an equal flow goes to both the RPA and the LPA, there is the least amount of energy loss in the flow. Furthermore, the energy loss in the flow increases the more the vessel diameters in the connection are different. Thus, energy loss in the flow is minimized with equal flow in the vessels.
The current Fontan connection typically resembles a mix of FIGS. 6 and 7, thus giving rise to substantial energy losses due to disadvantageous inlet flow collision. FIG. 6 illustrates a one diameter offset planar TCPC model. SVC and IVC diameters are more representative of the in vivo dimensions. FIG. 7 illustrates a TCPC model incorporating non-planar arrangement of pulmonary arteries, wherein FIG. 7(a) shows a front view, and FIG. 7(b) a top view of the same connection. The FIG. 7 model also incorporates a one diameter caval offset; however, all vessels are assumed to be the same size.
While, experimentally, the model of FIG. 5 shows good results compared to the others, nutritious blood coming from the hepatic veins goes primarily to one lung, increasing the size and functionality of this lung compared to the other lung, and this is not optimal.
It is evident that current anatomical connections provide non-optimal energy loss in the flow at regions of the connection where there is a high level of venous flow mixing and disturbance. There is thus a need for an anatomical connection providing optimized flow control, equal distribution of hepatic blood to both lungs, and minimized energy loss, that in effect minimizes or elimates inlet flow collision. It is the provision of such a connection that the present invention is primarily directed.